The secretory granule matrix is a miniature biopolymer that consists of a charged polymer network that traps peptides and transmitters when it condenses and releases them on exocytotic decondensation. Models of exocytotic fusion have treated this matrix as a short circuit and have neglected its electrical contributions. This matrix responded to negative voltages by swelling, which was accompanied by a large increase in conductance, and to positive voltages by condensing. Thus, the matrix resembled a diode. The swollen matrix exerted large pressures on the order of 12 bar. The responses took place within milliseconds of the application of the electric field. These findings suggest that matrix decondensation, and therefore product release, is controlled by potential gradients.